The tendency of modern electronic devices is toward a smaller size, a lower profile, and more functions, and hence various components such as various IC units used in such electronic devices are required to be smaller in size, lower in profile, and have more functions. In view of the tendency, it is desired that printed wiring boards on which the electronic components are to be mounted be smaller in size and higher in density. One approach to the requirement is to use a multilayer printed wiring board which has wiring patterns in a multiple of layers.
FIG. 5 shows a conventional method of manufacturing a multilayer printed wiring board. The conventional method of manufacturing a multilayer printed wiring board will hereinafter be described with reference to FIG. 5. First, prepregs 2, 3 are placed on the respective upper and lower opposite sides of a dielectric laminate (inner layer laminate) 1 which has conductive wiring patterns (not shown) on both of its both surfaces. Then, an outer layer material 4 composed of a copper foil is placed on the upper side of the prepreg 2, and an outer layer material 5 is placed on the lower side of the prepreg 3. The inner layer 1, the prepregs 2, 3, and the outer layers 4, 5 jointly form a multilayer (four-layer) printed wiring board as a first page. A multilayer printed wiring board as a second page, which is the same construction as that of the first page printed wiring board is positioned below the first page printed wiring board. Between the first and second pages, there are disposed parting films 7, 11 and a mirror finished steel plate 9 sandwiched therebetween. The multilayer printed wiring boards which are separated by the mirror finished steel plate are stacked ranging from 5 to 12 pages, and then are sandwiched by bonding dies 16, 17 with dummy plates 12, 13 and parting films 14, 15 interposed therebetween. The multilayer printed board stacks are then inserted between hot press plates, and pressed to laminate with heat.
In order to position the circuits on the inner layers into registry, both stacked laminates have punched through holes in the same positions, and are fixed in position by positioning pins extending through the through holes.
The above prior art is disclosed in Japanese Laid-Open Patent Publications Nos. 60-62193 and 60-65598.
The mirror finished steel plates serve to reduce thickness irregularities and minimize surface irregularities due to projecting conductive wiring patterns on the surfaces of the inner layers of the laminated multilayer printed wiring boards, and uniformize the temperature distribution in the direction in which the printed wiring boards are stacked. Therefore, the mirror finished steel plates are indispensable when the multilayer printed wiring boards are laminating with the conventional method.
The parting films are employed to prevent both the multilayer printed wiring boards and the mirror finished steel plates from being bonded due to a flow of interlayer adhesive (prepregs) while the stacks are being pressed to laminate with heat by the press plates.
The dummy plates are used to improve the temperature distribution in the direction in which the printed wiring boards are stacked, while they are being heated by the press plates.
With the conventional method, the mirror finished steel plates have to be thoroughly cleansed each time a heating and pressing cycle is finished, so that powdery foreign matter and resin refuse from the prepregs will not remain attached to the mirror finished steel plates, and the mirror finished steel plates thus cleaned are repeatedly used.
However, when the multilayer printed wiring boards are stacked after the mirror finished steel plates are cleansed, the outer layer materials (copper foils) are liable to be scratched or dented when they are pressed because powdery foreign matter and resin refuse from the prepregs are apt to be attached to the mirror finished steel plates, or due to surface irregularities of the mirror finished steel plates. If this happens, the wiring patterns on the multilayer printed wiring boards become defective.
According to the conventional buildup method, the parting films must be superposed on the upper and lower surfaces of each of the mirror finished steel plates. This procedure is however inefficient and complex, resulting in obstacles to product quality stability, cost reductions, and process automation. Specifically, since there are many steps involved in stacking the multilayer printed wiring boards, it is difficult to automate the process using robots.